Continuous casting method

ABSTRACT

The invention concerns the continuous casting of metals of all kinds by direct cooling with liquid metal or ionic liquid as coolant, for direct cooling of the strand ( 4 ). According to the invention, the coolant is directed to the strand ( 4 ) in at least one jet ( 10 ) and flows in a turbulent flow in this jet.

The invention concerns the continuous casting of metals of all kinds where liquid metal is used as coolant for directly cooling the strand and a device for such a cooling method according to the introducing parts of claims 1 and 9, respectively.

There are several documents in the prior art which show such a process:

The U.S. Pat. No. 3,430,680 A discloses a cooling method, with a vertically oriented pipe, through which the coolant flows due to gravity. A nozzle is centrally inserted into this pipe, the metal to cast runs through this nozzle in a liquid state and the same velocity as the coolant, in order to avoid any shear forces or other disturbances of the liquid-liquid contact surface. The metal to cast solidifies from this contact surface towards the centre of the strand and is finally hard and tough enough to be separated from the coolant. Beside the enormous difficulties to come to identical velocities for two separate liquids in a stream consisting of two components flowing in purely a laminar state, the cooling efficiency is rather poor for exactly this reasons!

The U.S. Pat. No. 3,874,438 A discloses a cooling bath of liquid metal which has its surface underneath the (shape providing) crucible outlet. The temperature situation is very tricky, the melt reaches its solidification point at the area of the outlet, shortly before entering the cooling bath. The cooling bath is provided in a cylinder and cooled by heat transfer through the side walls of the cylinder. Special provisions, namely an additional recipient for cooling liquid are provided for keeping the surface of the coolant at the chosen height. It is extremely difficult to keep the solidification temperature within the outlet, one has to take into account that the strand is still liquid over the greatest part of its cross section, that thermal energy is set free by the solidification and that the cooling process along the strand is changing during the cast, because the coolant gets warmer and warmer.

The U.S. Pat. No. 5,344,597 A discloses a very sophisticated process for the manufacturing of thin sheets of steel out of the liquid state: A thin layer of molten cast is brought to the surface of a coolant consisting of molten metal (e.g. led) and swims on it to a roller bed where it is kept, guided and moved away. The coolant comes into contact with the lower side of the cast only. The coolant is transferred from the surface region of the bath to a cooler and brought back to the bottom area of the bath by a pump. Due to the one-side-only cooling, due to the fact that the coolant is underneath the cast and due to the parallel movement of the sheet to the surface, the heat transport and therefore the cooling is rather poor and asymmetrical, leading to stress and distortions of the product.

A similar process is described in U.S. Pat. No. 4,955,430 A: The main difference is, how the liquid steel is brought onto the led and out of it again, the very casting process is the same and has the same setbacks.

Two very similar devices and processes, respectively, are disclosed in U.S. Pat. No. 4,510,989 A and in U.S. Pat. No. 4,751,959 A, respectively: Molten cast, which already has a thin solid layer on its surface, is brought into a cooling bath consisting of molten metal and moved in along a bow-like path. The volume of the bath is much, much bigger than the volume of the immersed strand therefore, an enormous amount of coolant is necessary for the process. The coolant is, at its best, randomly agitated by circulation pumps mounted on the bottom of the vessel. Great problems arise due to the big surface area of the coolant and the dangerous vapours and gases resulting from the various processes in the bath. An other problem lays with the fact that the strand has to be brought into a curved form and than into an elongated form again. This causes structural problems and flaws in the quality of the final product.

U.S. Pat. No. 2,363,695 A shows an interesting idea: Molten material, in most cases steel is fed through an thermally insulated pipe with U-shape to a nozzle which is directed upwards in a vessel filled with liquid led as coolant and then drawn vertically upwards. The coolant is kept in the vessel without stirring or agitating, therefore only moved by the strand and the convection movement due to temperature differences, meaning without remarkably movement. This, and the co-movement of the strand and the led raising due to its warming brings problems with uniform cooling and long lasting operation cycles.

SU 863 161 A discloses the casting of a pipe in two steps: In the first step, a water cooled mould is used to produce a thin layer of solidified metal on the surface of the strand, in the second step, the strand is, along a curved path, further cooled by direct contact with liquid metal. The liquid metal is kept in a ring-like slit around the cast and is cooled indirectly with water. Beside problems with the toroidal form of the mould, problems with the uniformity of complicated heat transfer exist: The heat goes from the molten core through the solidified surface area to the liquid metal which is used as coolant, further into the wall of the mould and into the water which circulates in channels in this wall. It is nearly impossible to come to a defined and uniform cooling scheme with such an arrangement.

U.S. Pat. No. 3,128,513 A discloses a casting process where molten salt is used as coolant. Therefore, the strand has a higher density than the coolant and sinks to the bottom of the vessel. The pressure of the liquid interior of the strand is used to form the cross section of the strand, but this also brings a lot of problems and even dangerous risks (outbreak of molten metal, etc.). The coolant is simply kept in the vessel without any agitating, in some embodiments, where movable moulds for the strand are used, the contact between the surface of the strand and the coolant is hampered.

JP 62101353 A discloses a conventional casting process for pipes. In order to form the interior surface correct and smoothly, a hollow core is inserted at the nozzle. The hollow core is cooled on its inside with molten metal instead of water, in order to prevent any danger of direct contact of water and molten metal (steam explosion) in case of an accident. There is no direct contact between the strand and the liquid metal.

Principally speaking, the introduction of continuous casting is a very effective way of producing semi-finished products. Products are rolling ingots, extrusion billets, strips and wires, sometimes also pipes, furthermore forging feedstock and in small amounts also thixotropic pre-material. Cast materials are aluminium, copper, magnesium, nickel and their alloys as well as steel. The high number of parameters influencing the casting process has led to the development of many different designs of moulds.

Usually the casting melt is cooled indirect by a mould as far as it is necessary to solidify a shell strong enough to carry the stresses at the mould exit and to resist a breakout of liquid casting melt. Short behind the mould exit the strand is cooled directly by water realised as film cooling or as spray cooling or a two phase cooling with water and air. The direct cooling stage ensures the solidification of the liquid core of the strand. Sometimes the second cooling stage is followed by a third one, a submerging in a water bath or a soft cooling stage by a flow of air.

The process variant with an indirect cooling stage in the mould and a following direct cooling stage with water or water and air is state of the art, but the disadvantage of this cooling concept is that friction between mould and strand causes damage of the new formed surface. Furthermore, the reheating of the strandshell induced by the arising air gap between mould and strand as consequence of solidification shrinkage is disadvantageous too. These two events are leading to defects like surface cracks (in case of too high friction), segregation and cell size variations, shell bending significant for the subsurface layer of a continuous cast strand. In any case the subsurface is very different from the core of the strand and therefore has to be machined off especially from rolling ingots. This means that an additional process step is necessary which leads to additional costs. One approach to reduce the thickness of the subsurface layer is the application of lubricant. Many different lubrication systems have been developed applying lubricant but also lubricant/gas mixtures to reduce the friction and the heat withdrawal in the mould, but a fully elimination of the subsurface layer was not possible. Another approach was to reduce the mould length in order to decrease the thickness of the subsurface layer requiring a better and therefore costlier process control system.

For the production of single crystals, a heated mould is used in the so called Ohno continuous casting process (OCC), the mould temperatures are higher than the melting point of the cast material in order to prevent nucleation at the mould wall and to ensure axial directional solidification. The necessary heat removal for this process is realised by direct cooling at only one position at a defined distance from the mould exit. Strands produced in this process are always single crystals with a very smooth surface. But the production of single crystals is not the aim of usual continuous casting, as the produced strands should be formable by rolling, extruding or forging or other cold or hot working process with isotropic properties.

Between these two process types (casting with cooled mould and casting with heated mould) lays the possibility to work with an insulating mould and to cool the strand by direct cooling only. This also promises a smooth, subsurface layer free strand when working with correct process parameters. As the active mould length is very short, this process requires a very fast, accurate process control system.

One feature most described processes have in common, the use of water as coolant, induces a more or less stable steam film on the cooled surface, depending on the surface temperature and the cooling water supply density. This leads to a very strong variable heat transfer coefficient during the direct cooling stage. Depending on the cooling concept, the properties of the cast material, the roughness of the strand surface, water supply density and water velocity as well as water temperature are decisive. But these parameters are influencing hot tearing, surface crack development and possible casting rate. Because parameters may change during the casting process, also the quality of the product may change.

The EP 063 832 discloses a concept for the “casting” of a probe which gets solidified in its mould and is therefore no real casting process, even less a continuous casting process.

The DE 41 27 792 discloses to cast a problematic probe into a pre-heated mould with special geometric properties, where a special form of solidification takes place. This is a casting process, but has nothing to do with a continuous casting process.

As one may see, there exists a great interest in a simple, reliable continuous casting process and device which avoids the mentioned disadvantages without loosing the advantages of the known processes.

In order to achieve this aim, the invention proposes to use one or more jets or streams of liquified metal or ionic liquids as cooling medium with turbulent flow and, advantageously, an insulated mould. This makes sure that no water steam film exists at the surface of the strand and that the coolant hits the strand in a defined way after a defined treatment. This guarantees that the cooling properties and characteristics are well defined and controllable.

Ionic liquids or designer liquids is the name for a group of salts composed of organic kations and mostly inorganic anions which have a melting point below 100° C. They may be used with the invention as long they do not decompose at the maximal working temperature of the process or react with the strand under the given circumstances. In the following description, they are in most cases not mentioned expressively, but always included when the term “molten metal” or “coolant” or the like is used.

The mould consists preferably of an insulating mould, which enables a solidification of the strand shell in the near vicinity of the mould exit. This is responsible for the prevention of many surface defects and the prevention of an unwanted subsurface layer. Solidification occurs by the influence of the direct cooling. The direct cooling uses a liquid metal like lead, tin, bismuth, gallium, indium or alloys of them as well as other liquid metals or alloys being liquid below the solidification temperature of the cast metal or alloy.

The feature of direct cooling in continuous casting with liquid metal ensures a very constant cooling behaviour, prevents, it this is wanted, oxidation of the new formed strand surface and eliminates the danger of explosions as a consequence of the use of water as coolant fully. Furthermore the hot tearing and cold tearing may be eliminated by the choice of the cooling metal and cooling metal temperature at the cooler entry and cooler exit. The produced strand is substantially free of the well known subsurface layer usually found in conventional continuous casting processes. The grain structure of the produced strands can be controlled by adjusting the coolant temperature.

In some cases, par example when casting aluminium, or some other metals, oxidation of the surface may be advantageous, because it gives a very well defined border to the coolant with respect to reactions and interactions between the coolant and the strand. In such cases, air or oxygen may be inserted at the downstream end of the mould, the mould exit (coquille), but upstream of the place(s) where the jet(s) hit the surface of the strand. A very simple way to achieve this is (when vertically casting occurs) to let a small annular slot between the coquille and the coolant distribution unit which slot has a connection to the ambient air. If necessary, more sophisticated supplies may be used.

Furthermore, it is possible to attach the casting machine to a rolling unit, since the strand exit temperature can be adjusted and hence will safe energy costs for reheating. In this process type no lubricant is necessary, this makes the process easier, cheaper but also increases the quality of the produced strand, as it is known that the lubricant also interacts and reacts with the hot strand surface leading to hydrogen enrichment and other surface defects.

The liquid metal as coolant can be directed onto the hot strand surface as continuous film or jet or as drops. The coolant distribution unit can be realised by a continuous slot around the strand perimeter but also may consist of slotted segments at different angles to the strand withdrawal direction. In order to increase the heat withdrawal it is possible to add direct cooling stages to ensure a higher heat transfer area leading to higher possible casting rates. The mould itself can have any cross section and be cylindrical or conical getting wider in casting direction. For lower casting rates it is also possible to realise the direct cooling step by submerging the coolant distribution unit and the hot strand into a bath of liquid cooling metal.

In general it also is possible to operate a conventional mould with a first indirect cooling step and a secondary direct cooling step with liquid metal as coolant, but in order to prevent known surface defects and the inhomogeneous subsurface layer the cooled mould length has to be very short.

It is possible to use the casting process according to the invention for vertical and horizontal continuous casting (or in any other desired angle). The invention was successful applied for casting of copper, magnesium and aluminium showing that it can applied for all non-ferrous metals and alloys as well as for steel.

Advantages of the new cooling concept are:

-   Easier cooling control, since the heat transfer number is very     constant in comparison to that of a direct cooling with water and in     comparison with the undefined submerging in stagnant coolants     consisting of liquid metals or salts. -   No oxidation of the new formed strand surface, if this is wanted -   Smooth surface without surface defects -   No or only a negligible inhomogeneous subsurface layer of the cast     strand exists - (no machining off necessary) -   Grain structure can be controlled by adjusting the coolant     temperature -   Hot and cold tearing can be eliminated by adjusting and controlling     the coolant temperature in the different stages of the cooling     stages as well as by the choice of the liquid metal (or alloy) as     coolant -   Inline rolling of the cast strand is possible and would safe energy     costs for reheating -   No lubricant necessary -   Easier mould design

The invention will be described in greater detail under reference to the drawing. The drawing shows in:

FIG. 1 a mould according to the invention in a vertical cross section,

FIG. 2 an other embodiment of the invention in a similar view,

FIG. 3 a third embodiment of the invention in a similar view,

FIG. 4 a fourth embodiment of the invention in a similar view,

FIG. 5 a fifth embodiment of the invention in a similar view,

FIG. 6 a sixth embodiment of the invention in a similar view,

FIG. 7 a principal view of the cooling system and

FIG. 8 a principal view of an strand cooler.

FIG. 1 shows a strand with vertical withdrawal direction. The cooling is done in a totally new way, using a complete filled strand cooler which is, in some ways, operated similar to heat exchanger known from chemical industry. The melt 1 is sucked from the tundish 2 (which can be heated) into the mould 3 and solidifies at the mould exit since the strand 4 is cooled by a liquid metal coolant 8 over the entire length of a cooling unit. The coolant 8 fills the entire gap-like room 11 between the surface of the strand 4 and the inner surface of a pipe 12 which surrounds the strand. The temperature of the strand 4 decreases during its movement through the strand cooler until its end is reached. A strand cleaning unit 7 ensures the slip off of the coolant from the strand 4.

But contrary to common heat exchangers, the cold coolant is fed into the strand cooler 5 and is distributed as it is required for the cast shape by a coolant distribution unit 6. The coolant 8 leaves the coolant distribution unit 6 either through a slit which has the form of a ring (depending on the cast shape) and is directed to the surface of the strand 4 or through a plurality of openings or nozzles which are arranged along a closed line and are directed to the surface of the strand too. The variant with the slit forms a closed, conical “wall” of flowing coolant 8, the variant with the openings a plurality of jets 10 of coolant 8. In both cases, it is important that the velocity of the coolant 8, when leaving the coolant distribution unit 6, is high enough to make the flow turbulent. The reason for this is, that a turbulent flow has a much greater capacity of heat transport in the direction normal (away from the strand) to the flow direction than a laminar flow.

The situation in FIG. 1, with the wall or jets 10 flowing in their very media, means that in some distance from the coolant distribution unit 6 the combination of the movement of the strand 4 with the movement of the coolant- 8 due to its circulation, forced by a pump as described later, overcome the jets 10 or wall and the flow pattern in the coolant 8 induced by them.

From the mould exit to a coolant collecting unit 9, the coolant 8 takes up heat from the hot strand 4, thereby heating up. The coolant collecting unit 9 ensures the required coolant distribution along the strand perimeter. This process type enables highest cooling rates but needs an accurate pressure control in the coolant feed.

In order to explain the discrimination between laminar and turbulent flow, reference is made to FIG. 8, which shows a slit of circular form between two concentric, circular walls, the inner cylinder having an diameter of “d”, the outer, hollow, cylinder an inner diameter of “D”, the hydraulic diameter “D_(H)” of the slit is calculated by: D _(H) =D−d

The hydraulic diameter “D_(H)” of an annulus and of noncircular channels is equivalent to the diameter of a circle with the same cross section. Hydraulic diameters for different shapes of the cross section are listed for example in Robert H. Perry (Ed.): “Perry's Chemical Engineers' Handbook” (Sixth Edition 1984). Pages 5-25 and 5-26 of this publication are hereby incorporated by reference. If a liquid medium flows through the slit with the hydraulic diameter “D_(H)” (given in meters) in the direction normal to the plane of the drawing, having a kinematic viscosity v (given in m²/s) and the mean velocity v (given in m/s), the so called Reynolds number Re may be calculated by: Re=(D _(H) ·V)/V

The point, where laminar flow turns over to turbulent flow is not only dependent on the cross-section of the channel but also on the shape of the cross sectional area. For Reynolds numbers (which have, by definition, no dimension) greater than about 12 000 in the case of channels in form of annular slits, the flow is usually turbulent. According to the invention, the Reynolds number in case of a “wall jet” should be at least 15 000 and preferably over 25 000. It has to be mentioned, that the slit in the coolant distribution unit 6 is not cylindrical, but conical, but the differences are small enough to be disregarded.

In a preferred embodiment the coolant distribution unit 6 comprises several individual parts, which can be adjusted against each other preferably by means of a thread, in order to change the width of the conical slits in the coolant distribution unit 6. This enables the operator to change easily the width of the slits, and thus the Reynolds number, even during operation.

In case of discreet openings or nozzles with the free diameter d (in meters), the Reynolds number is defined by: Re=(d ·V)/V

The change from laminar to turbulent flow occurs with such a geometry at a point between 2 600≦Re ≦4000, depending of hard to define second order effects. The Reynolds number in case of such individual jets should be therefore at least 5 000, preferably over 7500.

For all usable coolants liquid metals as well as ionic liquids, the kinematic viscosity may be found in the data sheets or chemical or metallurgical textbooks, the velocity is given by the known cross section area (in m²) of the slit and the volume of coolant (in m³) passing per second, the width of the slit (which is half its hydraulic diameter) is known from the construction, therefore, with this description at hand, there exists no problem for the man skilled in the art to come to the turbulent flow which is used by the invention.

FIG. 2 represents a process type, in which the cast strand 4 may be cooled softer than in the process type of FIG. 1. The casting melt 1 is sucked from the tundish 2 (which can be heated) into the mould 3 and solidifies at the mould exit as the heat is withdrawn by the coolant in direct contact with the strand 4. Instead of a pipe 12, a cooling box 13 is provided around the area where the strand 4 solidifies during its movement. The cooling box 13 serves to collect the hot coolant. At the bottom end of the cooling box 13 a strand cleaning unit 7 is fixed, it ensures that no coolant (in a technical sense) is remaining on the strand surface. The “cold” coolant is distributed along the strand perimeter as required for the cast strand shape by a coolant distribution unit 6. After getting in contact with the strand 4, the now hot coolant flows down to the bottom of the cooling box 13 and then leaves it through the coolant outlet.

FIG. 3 represents a casting process according to the invention, and mould, respectively, with a heat withdrawal rate, which is substantially higher than that of the aforementioned casting processes shown in FIG. 2. Due to two consecutive cooling steps a high rate of heat flow away from the strand 4 to the coolant 9 is achieved. Thereby separate coolant feeds are provided for each cooling step. The casting melt 1 in the tundish 2 (which can be heated) is sucked into the mould 3 and solidifies at the mould exit. The axial heat removal in the strand 4 is, in a first cooling stage, similar to that according to FIG. 2 but gets increased by a second cooling stage in an additional cooling unit, which is similar to the cooling unit shown in FIG. 1. The device for the first cooling stage consists of a coolant distributor 6 which produces a coolant film 14. The device for the second cooling stage consists of a coolant distribution unit 6′ and an attached pipe 12, acting as a heat exchanger tube, which ensures a higher heat removal than cooling stage one. The strand 4 is cleaned (technical clean) from the remaining coolant 8 on the surface by the cleaning unit 7. A cooling or collecting box 15 encloses the whole cooling unit.

The FIGS. 4, 5 and 6, respectively, show devices similar to those shown in FIGS. 1, 2 and 3, respectively, but with horizontal withdrawal of the strand. Continuous casting with horizontal withdrawal is well known in the art, for the person skilled in the art, there is no problem to adapt the invention to this version of casting. The only difference that should be mentioned is, that the liquid metal has a much higher density than the water which has mostly been used in the prior art. Therefore, the free applied coolant in the devices according to FIG. 5 and the first cooling stage of FIG. 6 must be differently pressurised on the top-side and the down-side of the strand 4.

FIG. 7 shows the flow sheet for the whole casting plant: The liquid metal used as coolant is stored in a tank 16, which needs to be heated by a heating unit 17 before starting the casting process. The liquid coolant is pumped by the pump 18 into the cooling unit 5. In the cooling unit 5 it picks up heat from the hot strand 4, then the hot coolant leaves the cooling unit and gives up this heat in the heat exchanger 19. Then the cold coolant flows back into the coolant tank 16. The heat withdrawn in heat exchanger 19 can be used for different things in any case it may help to safe costs for energy in a firm. The coolant tank 16 as well as the whole cooling system needs to be free from air and especially from oxygen, this is ensured by flushing the coolant tank 16 and the cooling unit 5 with inert-gas 20. As inert-gas 20, all such gases known in the art are usable, they have to stay inert at the given temperatures at contact with the coolant and the material of the strand. It is, of course, advantageous to use the same inert-gas in the storage tank 16 and the cooling unit 5. The whole casting plant can further comprise a strand withdrawal unit 25 and a Flying saw 26 for cutting the strand 4 in pieces of certain length.

In order to come to defined and repeatable conditions in the cooling unit 5, it is preferred to have sensors for the temperature (TIC) 21, 22, sensors for the flow rate (FIC) 23 and sensors for the pressure (PIC) 24 at least near the entrance of the cooling agent into the cooling unit 5. It is of course advantageous to have further measuring points within this system.

The invention is not restricted to the shown and described embodiments.

Coolant can be a liquid metal like lead, tin, bismuth, gallium, indium or alloys of them as well as metals or alloys, which are having a melting point lower equal 60% of the melting point of the casting material. Further, it is possible to use non-metallic liquids, namely any liquid medium, which does not react with the material of the strand at the relevant temperatures and which stays in a liquid state at all temperatures involved in the cooling process. This may be some organic compounds, especially for strands of low-melting alloys.

It is not necessary that the storage tank 16 is arranged at lower level than the mould 3, but for safety reasons, this arrangement is preferred. If an other arrangement is provided, the pump 18 and other armatures have to be put to other positions, but this brings no problem to the man skilled in the art.

The pipes, the pump 18, the armatures, the sensors 21, 22, 23, 24, the cooling device 5, the pipe-like heat exchanger and other equipment for the coolant are, given the disclosure of the invention, readily available for the man skilled in the art of casting metal, may it be ferrous or not.

Some additional features and advantages of the invention are: The casting process can apply one or more direct cooling steps. The use of liquid metal as coolant prevents, if this is wanted, the formation of oxide layers on the strand surface. The adjustment of the coolant feed temperature and coolant flow rate allows good control of the cooling rate and hence the formation of grain structure. The use of an insulating mould or, more precisely, a low heat removal in the mould, prevents the formation of surface defects and inhomo- geneous subsurface layers. The use of liquid metal for the direct cooling in continuous casting eliminates the danger of explosions known from the conventional process using water as coolant. This increases the safety in cast shops enormous. For this continuous casting process no lubricant is necessary. Applying one of the above described process types in horizontal continuous casting enables inline rolling of the cast ingots in order to safe energy costs for the reheating of the ingot. The process eliminates hot tearing and cold tearing when operating at optimum process parameter (coolant temperatures at different stages of the cooling unit). The process has no restrictions concerning the shape of the cast strand or the number of parallel cast strands.

The existing plants may easily be adapted to the invention, existing cooling systems using water my be stripped and replaced by the new system. The mould itself hardly needs any adaptation, it is only necessary to have the freezing area at the end of the mould, therefore, insulated moulds or very short cooled moulds may be best used. 

1. A process for continuous casting of metals, whereby liquid metal or ionic liquid is used as coolant for direct cooling of the strand, characterized in that the coolant is forcibly directed at the strand in at least one jet with turbulent flow.
 2. The process according to claim 1, characterized in that the coolant is chosen from the group consisting of: lead, tin, bismuth, gallium, indium or alloys thereof.
 3. The process according to claim 1 characterized in that the coolant has, in Centigrade Celsius, a melting point which is lower or equal to 60% of the melting point of the casting material in Centigrade Celsius.
 4. The process according to claim 1, characterized in that the at least one jet streams into coolant filling an entire gap-like room between a surface of the strand and an inner surface of a pipe which surrounds the strand.
 5. The process according to claim 1, characterized in that the coolant flows essentially in the direction into which the strand is moving.
 6. The process according to claim 1, characterized in that the jet has the form of a conical wall jet and in that its Reynolds number amounts to at least 15 000 and preferably to over 25
 000. 7. The process according to claim 1, characterized in that the jet comprises a plurality of individual jets with circular cross section and in that its Reynolds number amounts to at least 5,000.
 8. The process according to claim 1, characterized in that oxygen or an oxygen containing gas, is supplied to the strand upstream of the point where the jet(s) hit(s) the strand.
 9. A device for a process according to claim 1, with a storage tank for the cooling medium, a heating element and a pump, with pipes which connect the storage tank with a cooling device for the strand and eventually a heat exchanger which is located in the pipe transporting the coolant from the cooling device to the storage tank, characterized in that the cooling device includes at least one nozzle which directs the cooling liquid directly onto the strand, preferably in near vicinity of the mould exit, and a coolant collecting unit.
 10. The device according to claim 9, characterized in that the cooling device (5) includes a pipe (12) arranged around the strand (4) or its path, respectively, and forming a gap-like room around the strand (4) which is filled with coolant (8).
 11. The device according to claim 9, characterized in that at least one, coolant distribution unit is provided in near vicinity of the mould exit, combined with a pipe arranged in some distance from the at least one coolant distribution unit (6) in the direction of the movement of the strand and having a second coolant distribution unit on the upstream end of pipe.
 12. The device according to claim 9, characterized in that a cleaning unit for the surface of the strand is provided.
 13. The device according to any of the claim 9, characterized in that the mould is an insulated mould.
 14. The device according to claim 9, characterized in that the nozzle has the form of a ring-like slit surrounding the strand.
 15. The device according to claim 9, characterized in that a plurality of nozzles are arranged along a ring-like line surrounding the strand.
 16. Device The device according to claim 9, characterized in that an inlet for oxygen or an oxygen containing gas, is provided between the mould exit and the nozzle(s) for the coolant.
 17. The device according to claim 12, characterized in that the cleaning unit is outside the cooling device. 